1. Field of the Invention
This invention relates to digital imaging devices, and particularly to on-chip black normalization for digital imaging devices.
2. Description of the Related Art
Digital imaging has become popular in recent years, including digital cameras, digital camcorders and high definition televisions (HDTVs). Most digital imaging systems use an image sensor that converts visible light into an analog or digital signal. Many image sensor applications, especially video applications, require a signal level to represent the absence of visible light (the color black). The signal level that corresponds to black needs to be stable over varying temperatures and illumination conditions so that the image sensor delivers consistent color quality.
Black normalization, or black clamping, is a process used by imaging systems to maintain an accurate black reference over time. Some of the sensor's pixels are masked to generate a signal that corresponds to black. This signal is used to update and maintain a black reference for the overall sensor.
Black normalization is more important for video or HDTV applications because it has to be performed almost instantaneously and continuously. It is less critical for still photography applications because normalization of all colors can be accomplished at some time after the image has been captured.
An image sensor currently used by many imaging systems is a charge-coupled device (CCD). Black clamping in CCD imaging systems has been accomplished separate from the image sensing chip, as shown in STMicroelectronics' data sheet STV0502. Therefore, CCD imaging systems occupy a relatively large amount of space on a circuit board and are expensive to manufacture. As shown in the same data sheet, black clamping is done with analog components, which are difficult to control and are limited in the calculations that can be performed during the normalization process.
Another type of image sensor employs CMOS sensing elements. CMOS sensors use a fixed black reference or are used in applications that do not need a continuously updated, accurate black reference.